1. Field of the Invention
This invention relates in general to apparatus for guiding a tape through a tape path in a tape recorder; more particularly, the invention is concerned with tape guides for changing the direction and inclination of the path of a magnetic tape in a magnetic tape recorder.
2. Description Relative to the Prior Art
While it is not so restricted, the invention acquires a special significance when it is used in a video tape recorder to guide a magnetic tape toward and away from a helical recording drum carrying one or more rotating heads. The tape is curved around the recording drum so that it emerges from the drum at a different level from that at which it entered, describing a helical path as it travels around the drum. With the tape accordingly placed around the drum, a television signal train may be provided to the heads and helically recorded on the tape. However, the helical recording format requires precise positioning of the span of tape which passes around the drum; positioning errors may cause, for example, mistracking during playback. Tracking problems are diminished therefore, if the tape follows a precise path into, around, and out of the drum assembly.
While the tape guide of this invention is generally applicable to tape recorders, it is particularly well adapted for use with one frequently used video recorder configuration in which the tape is initially pulled from a coaxial-reel cassette and wrapped around the recording drum. In this configuration, the paths into and out of the drum are horizontal and substantially in the same plane as supply and takeup reels coaxially mounted in the cassette. The last post before the tape touches the drum and the first post after the tape leaves the drum are designed to change the tape's horizontal level a few degrees so as to dispose the tape properly to form a helix around the drum.
Tape guides have been proposed for changing the direction and inclination of magnetic tape within a coaxial-reel cassette. One form of guide is exemplified by U.S. Pat. No. 3,684,208. In their most common form, these guides include rotatable truncated cones for guiding magnetic tape along an inclined path from the level of one coaxial reel to the level of the other coaxial reel. The taper of each cone causes the tape to incline in the direction of the increasing radii of its conical surface. Since the tape tends to move transversely along the cone's surface, flanges are formed integrally with the cone to restrain the tape and prevent it from riding off the guide. These flanges must rotate with the cone; this means the flanges must be so disposed with respect to the rotation axis of the guide that the tape contacts the flanges only during a certain portion of its angular wrap around the rotating guide. Otherwise, if the line of contact varies, the tape will be intermittently bumped and distortions or vibrations will be established along the length of the tape. This situation is most simply avoided by mounting the flanges so that the planes that they describe are perpendicular to the rotation axis of the guides; in other words, the flanges are normal, and not contoured, with respect to the axes of the conical surfaces.
The guides thus far described have edges which do not contribute significantly to changing the inclination of the tape path. Edges which do contribute are illustrated in U.S. Pat. No. 3,263,936.These guides, representing an alternative approach to guiding a tape past a tapered surface, provide contoured guide surfaces to achieve the level offset between two coaxial reels. Edges, integrally formed on the guides, locate the tape in a vertical direction and follow the inclination of the tape as it rounds the guide surface. However, to prevent the vibration or distortion mentioned above in connection with rotating guides, the guides and their associated edges must both be rigidly mounted in the cassette so as not to rotate with tape movement. Otherwise, intermittent vibration of the tape would render them useless.
Apparatus has been devised for automatically wrapping a tape around a drum of a video cassette machine. The cassette, containing the tape inside, is first inserted into the machine. Generally, a tape door is automatically opened to allow one or more tape threading guides to be placed within the cassette and close to the tape. Each tape threading guide is adapted to engage the tape within the cassette and to pull the tape out of the cassette and toward the recording drum. The tape is then guidedly curved around the drum so that it forms a helix angle with the arcuate path of one or more recording heads. Each tape threading guide has a guiding surface with an orientation that encorages the tape to form a helix around the drum. For example, in U.S. Pat. No. 3,665,120, a guide having a conical outer surface is rigidly mounted on a movable support plate for engaging a magnetic tape within a cassette and for pulling the tape out and around a recording drum. The conical surface is sufficiently tilted so that the tape is held against its tendency to laterally drift along the guides as it changes its direction and inclination of travel.